Study On Moist Physics Process Parameterization And Dynamics Of Convective Precipitation Over The Tibetan Plateau

Due to the unique thermal and dynamic effects in summer over the Tibetan Plateau(TP),the basic features of the convective precipitation process in this region are special.For current mesoscale numerical models,the simulation of clouds and convective precipitation process over the TP is still uncertain,in which the moist physics process(cumulus convection and cloud microphysics)parameterization closely associated with clouds and convective precipitation process both play the important roles.In this paper,the performance of convective precipitation simulation is evaluated with the Weather Research and Forecasting(WRF)model.Based on the precipitation biases and errors,a new convective adjustment time scale for the Kain–Fritsch eta(KFeta)scheme in the WRF model is proposed.The connections between the new convective adjustment time scale and characteristics of convective clouds are analyzed,and the performance with the new scheme on the precipitation simulation are preliminarily validated.The influence mechanism of deviation in initial reanalysis condition on the convective precipitation simulation in TP is analyzed as well.The performance of simulated upper-level ice clouds using the WRF double-moment 6-calss(WDM6)microphysics scheme is investigated compared with observation.A modified parameterized ice phase process scheme based on WDM6 scheme is proposed and the preliminary validation experiment is carried out.In addition,the role of surface diabatic heating and effect of condensation latent heating released by precipitation on TP vortex(TPV)associated with precipitation as well as the relationship between gravity wave dynamic characteristics and precipitation are analyzed by simulating several precipitation processes over the TP.The major contents and results are as follows:In this paper,the simulation of a typical convective precipitation case over the TP is conducted to compare with the Automatic station and the Climate Prediction Center's morphing technique(CMORPH)hourly precipitation fusion products.The results show that the large deviation occurs in the original KFeta scheme,and a little improvement occurs in Multi-Scale KFeta(MSKF)scheme.The analysis indicates that the original KFeta scheme does not comprehensively consider the characteristics of weak and deep convection over the TP,which may be not suitable for the convective process over this region.A new convective adjustment time scale is thus proposed,which is not dependent on the horizontal resolution,considering more about vertical motion process within cloud and can be implemented in high-resolution simulations.The new convective adjustment time scale reduces the settling time of convective instability to a quasi-equilibrium or neutral state over this region,performs reasonably well in representing the strong entrainment processes and the increasing accumulated precipitation,largely improves the simulated precipitation pattern,horizontal scale and intensity.The simulated convective dynamic process rapidly performs a convective adjustment for the deep convection with weak convective available potential energy(CAPE)though changing the temperature tendency feedback.And the applicability of the new scheme is confirmed through simulating three cases of convective precipitation.The results also show a vital fact that the RH bias in the lower level atmosphere of the initial condition has a significant influence on the propagation of the convective precipitation system over the TP region through the convective feedback mechanism of the parameterized lower levels cooling by several sensitive tests.The correction to RH in the convective parameterization scheme(CPS)strengthens the downdraft depth and convection initiation near the surface,and therefore reduces the bias of convective propagation to some extent.The improper description for simulated clouds habits in cloud microphysical scheme will influence the simulation of cloud and precipitation to some extent.Comparison between synthetic satellite imagery and observed FY-2E imagery shows that the WDM6 microphysics scheme produces the warm biases for cloud top brightness temperatures(BTs)of deep convective clouds system.Further analysis by sensitivity experiments demonstrates that these biases are mainly sourced from the unrealistic parameterization of upper troposphere ice clouds process,in which simulated ice crystals with larger volume-weighted mean diameter near the tropopause sedimentate more quickly,thus ice content is significantly reduced,leading to the warmer cloud top BTs.This paper presents a modified ice phase process parameterization method,in which ice crystals number concentration can be related to temperature,saturated vapor pressure and vertical movement and introduce a temperature-dependent mass-dimensional relationship to descript the properties of ice cloud particle.The revised treatment improves the performances of ice phase process and precipitation pattern.Moreover,the applicability of it is preliminarily verified by simulating a convective precipitation event over TP.The diabatic heating plays a crucial role in the process of convective precipitation over the TP,which is also an issue worth further study.For the study of thermal forcing over the TP,three cases of TP vortex(TPV)with heavy precipitation are simulated with the WRF model.The sensitivity experiments with and without surface diabatic heating and atmospheric latent heating released by precipitation are conducted.Results show that,before the genesis of the TPV,there exists an anomalous increment of surface heating flux;while a disturbance of potential vorticity(PV)at 500 hPa moves over this region,the PV is enhanced and induces the generation of TPV.The occurrence of precipitation in turn reduces the surface heating flux.The condensation latent heating release caused by precipitation during the development of TPV does not have further contribution to the enhancement of TPV.The dynamic diagnoses of local PV further indicate that,in the early phase,the strong diabatic heating in the lower levels(below 500 hPa)associated with the increasing surface heating flux has a significantly positive contribution on the generation of the TPV.Later,the occurrence of precipitation resulted in the decline of positive diabatic heating contribution at low levels.Inversely,the middle levels(above 475 hPa)diabatic heating associated with the released latent heat of precipitation condensation tends to dominate the negative contribution to TPV,resulting in the decay of TPV finally.Overall,the evolution of TPV is more like a conversion from surface heating flux to atmospheric condensation latent heating.In the study of the dynamic mechanism of convective precipitation over the TP,the characteristics of gravity wave have an important influence on it.For the study of dynamic process,two heavy rainfall events in the eastern part of TP are simulated by using the WRF model.The results show that the propagation characteristics of gravity wave are represented by the alternative movement of vertical velocity and divergence respectively,and the ascending movement is closely associated with the development of precipitation.The analysis of the Richardson number show that vertical shear instability at 5km to 6km appears before the waves occur,with the signal of waves intensifying,the vertical shear start to weaken.It is shown that the gravity wave can extract energy from the unstable airflow of vertical shear.The mesoscale gravity waves in two heavy rains are identified by using the Morlet wavelet method,the gravity waves which have longer wavelength are generated early and disappear late,and which plays a major role in reinforcing the precipitation processes.